As disclosed in Japanese Patent Publication No, 2003-203795, a discharge lamp ballast for a discharge lamp, especially for a fluorescent lamp of hot-cathode type is configured to provide a preheating mode for preheating filaments, a starting mode for applying a high voltage after the preheating mode to start the lamp, and thereafter a lighting mode for rated lighting or dimmed lighting of the lamp. The duration of the individual modes is given by use of a timer. The discharge lamp ballast device includes a chopper for boosting a DC power give by rectification of an AC power from an AC power source, an inverter for converting the DC power output from the chopper into an AC power, and a resonance circuit which resonates the high frequency AC power output from the inverter to apply the same to the discharge lamp. The inverter includes a switching element of which switching frequency is varied so as to apply different voltages to the discharge lamp respectively during the preheating mode, the starting mode, and the lighting mode.
The discharge lamp ballast device is provided with a reset means which is configured to detect the output voltage from the chopper to the inverter in order to reset the inverter back to the preheating mode when the DC output voltage to the inverter is lowered due to an instantaneous power failure of the AC power source, thereby protecting the discharge lamp as well as circuit components of the inverter from undue stress.
Also, the discharge lamp ballast device is configured to stop the inverter upon detection of a lamp abnormality such as a no-load or a lamp life-end condition, for protecting the circuit components from undue stress.
Further, in order to avoid the inverter from being reset to the preheating mode or the starting mode immediately after the lamp start in case the output voltage from the chopper to the inverter is instantaneously lowered due to ripple voltage in the output of the chopper, the discharge lamp ballast device is configured to disable the reset means during the preheating mode and the starting mode, prohibiting the preheating mode even upon lowering of the output voltage to the inverter.
However, when the discharge lamp comes to its near lamp-life end, it is likely that the high lamp voltage results in an excessive load power which lowers the output voltage from the chopper to the inverter. With this consequence, the reset means operates immediately after the lamp start to resume the preheating mode or starting mode, and therefore repeat the preheating mode and the starting mode, thereby giving an excessive stress to the circuit components, and even resulting in a failure of the discharge lamp ballast device. Especially, when there is a great difference in the switching frequency between the starting mode and the lighting mode so that the inverter output varies to a large extent (for example, in a dimming of the lamp), the output voltage from the chopper to the inverter may be lowered instantaneously, which triggers the reset means during this transition period.